The invention relates to the provision of metering plates together with impingement inserts for use in gas turbine nozzles.
Gas turbine nozzles typically use impingement inserts inside of the nozzle to cool the airfoil walls. If the nozzle has a multiple circuit cooling system then there may be unbalanced cooling flow to the different circuits of the nozzle.
To overcome the problem described in the prior art, metering plates are used with or without impingement inserts to balance cooling flow to the different circuits of the nozzle. In one embodiment of the invention, a metering plate with a single metering hole is used.
In a second and preferred embodiment of the invention, a metering plate is used with multiple holes to overcome potential flow disruption which can be caused by a single metering hole. More specifically, when using only one metering hole in a metering plate a flow disruption occurs that produces a variable static pressure distribution in the area just below the metering plate. This variability in static pressure distribution relative to the rest of the impingement insert can cause variable impingement pressure ratios across impingement holes leading to back-flow issues and/or reduce cooling effectiveness. This flow field disruption is produced by the Vena Contracta of the orifice. Using several metering holes instead of just one significantly reduces the static pressure variation downstream of the metering plate.